Quantum confined or low-dimensional devices such as quantum dots and quantum wires are becoming increasingly important for use in potential applications in various fields. Whereas quantum dots provide quantum confinement of electrons and/or holes in all three spatial dimensions, quantum wires provide quantum confinement of electrons and/or holes in two spatial dimensions but allow electron transport in the third dimension. The unusual quantum properties exhibited by quantum dots and quantum wires make them useful candidates for incorporation into many future technologies. For instance, quantum dots may be used for many applications including light emitters and especially single photon sources. Single photon sources are expected to form an integral part of a plethora of future applications including quantum cryptography and quantum computing technologies. Quantum wires have been conjectured for use in numerous applications including transistors, integrated circuitry and charge sensing as well as quantum computing.
A number of mechanisms have been shown to produce optically active quantum dots with properties similar to natural atoms or trapped ions. Examples include self-assembled quantum dots formed by growth of epitaxial layers having a mismatched lattice constant, or atomistic defects in silicon carbide or diamond. Similarly, different techniques exist for forming quantum wires including use of naturally occurring crystals such as zinc oxide. However, each of these mechanisms face similar challenges when looking to incorporate quantum dots or quantum wires into devices for specific applications.
First, scalability remains a problem for large-scale manufacture of devices. In particular, the low yield for formation of quantum dots and quantum wires using many known techniques means that manufacturing devices on a large-scale, especially using diamond, is not yet practical. Furthermore, the position of a quantum dot or quantum wire on a substrate cannot be easily pre-determined or designed using most established methods of formation. Such a requirement will be necessary for feasible large-scale manufacturing of integrated circuits including quantum dots and quantum wires. In view of this, there is a need for quantum-confined devices such as quantum dot or quantum wire devices that can be produced on a large scale whilst offering deterministic positioning of the quantum dots or quantum wires on a substrate of choice. Moreover, for use of quantum dots devices as a single photon source, the quantum dots produced must also offer competitive spectral characteristics such as good uniformity and brightness.
The unique crystalline structure of layered two-dimensional (or two-dimensional layered) materials such as graphene and tungsten diselenide (WSe2) offer exciting new opportunities for quantum technologies. Two-dimensional materials exhibit a number of unusual properties compared to traditional three-dimensional semiconductor materials such as silicon (Si) or Gallium Arsenide (GaAs). In a two-dimensional material, atoms form in monolayers that are chemically bonded within the plane of the monolayer or sheet, but in which the layers are held together only through van der Waals forces. Each monolayer is naturally passivated without any dangling chemical bonds, and so no chemical bonds are formed between monolayers. Importantly, electrons, holes and many-body complexes are confined within the plane of each single monolayer and so provide an inherently two-dimensionally confined system. Quantum confinement of electrons in the two-dimensional plane leads to novel and unusual electronic and optical properties that are distinctively different from three-dimensional materials. Techniques for separation of one or a few monolayers of two-dimensional materials have been developed in recent years, thereby allowing researchers to study and develop devices in the two-dimensionally confined systems offered by layered materials. Further discussion of two-dimensional materials for nanophotonics technologies can be found in “Two-dimensional material nanophotonics” by Xia et al., Nature Photonics, 2014, vol. 8, pg. 899-907.
Naturally occurring quantum dots have been observed in layered materials such as tungsten diselenide (WSe2). Such quantum dots have been recorded particularly at the edge portions of the monolayers. Some reports have attributed quantum dot formation to crystal imperfections at the edge regions, while others claim they might be generated by atomic defects. Nevertheless, these naturally occurring quantum dots have not been reliably generated, or predictably positioned, which means than incorporating such dots into devices on a larger scale would be unworkable.
Therefore, there is a requirement for a quantum-confined device (such as a quantum dot device or quantum wire device) and its method of manufacture that is feasible on a large scale, robust and that offers deterministic placement of the quantum dots or quantum wires on a substrate. Furthermore, the generated quantum dots must have high quality optical and electrical properties.